examples: support all client endpoints

[zchee]

Motivation

• The example CLI only demonstrated GetOrgDevices, which made it hard to exercise and validate the rest of the ABM client surface.
• Provide a single, discoverable example program that can call every exposed ABM endpoint for development and testing.

Description

• Added an -endpoint driven CLI with constants for each supported endpoint and a runEndpoint dispatcher that covers all client methods (org devices, single device, AppleCare coverage, MDM servers, MDM-server device linkages, assigned-server linkage/details, create activity, get activity, and paginated part-number fetch).
• Added endpoint-specific flags (-org-device-id, -mdm-server-id, -org-device-activity-id, -fields, -limit, -activity-type, -activity-device-ids) and an optional -api-base-url override, plus usage text that documents supported endpoint values.
• Implemented request assembly for create-org-device-activity (relationships payload builder) and a shared CSV parsing helper (splitCommaList), and kept all endpoint outputs as formatted JSON.
• Created and recorded an ExecPlan at .agent/PLANS.md describing the work performed and updated the example to use golang.org/x/oauth2 TokenSource when creating the client.

Testing

• Ran gofmt -s -w examples/main.go successfully.
• Ran go test ./... and all tests passed.
• Attempted gopls version in this environment but gopls is not available here.
• Saved the ExecPlan to .agent/PLANS.md to track the implementation steps; commit was made with --signoff because a GPG signing key was not available in the environment.

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Codex Task